The present invention relates to a composition of matter usable to form liposomes comprising antifungal polyene macrolides and the production thereof.
Clinical observations and animal experimental studies have added to the understanding of host-fungal interactions. It is becoming recognized that host defense against fungal disease is multifactorial and may vary, depending on the etiologic agent. The mechanisms of resistance are not well defined in most instances, but various innate barriers and cell-mediated immune responses seem to be of primary importance. Clearly, debilitation of innate defenses and of cell-mediated immune responses can increase an individual's susceptibility to severe fungal disease from opportunistic agents such as Cryptococcus neoformans and species of Candida and Aspergillus, as well as from fungal pathogens such as Histoplasma capsulatum and Coccidioides immitis. The difficulty in gaining a complete understanding of the critical host defenses has been further complicated by many studies that show fungi may affect various host immune functions adversely. Although it is too early to evaluate the clinical importance of many of these experimental findings, investigators have demonstrated that fungi impair neutrophil function, induce IgE responses, and cause suppression of cell-mediated immune responses.
Host changes likely to be associated with increased susceptibility may be accidentally induced, as in traumatic injuries (such as burns or puncture wounds); selfinduced, as in chronic alcoholism; naturally occurring, as in diabetes mellitus, various congenital immune deficiencies, collagen diseases, lyaphoreticular neoplastic disease, and other types of tumors; or iatrogenically induced by instrumentation (such as catheterization), surgical procedures (such as open heart surgery), or by use of cytotoxic drugs (as in an attempt to prevent graft rejection and to treat neoplastic disease), corticosteroid therapy, and long-term use of broad-spectrum antibiotics.
Chemical factors that aid resistance to fungal diseases are poorly defined. Knowledge of these substances is based primarily on circumstantial evidence at the clinical level and in vitro observations at the experimental level. Hormonally associated increases in lipid and fatty acid content on the skin occurring at puberty have been correlated with increased resistance to tinea capitis caused by the dermatophyte Microsporum audouinii, although pubescent changes are not the sole factors in resistance. Substances in serum, cerebrospinal fluid, and saliva may limit growth of Cryptococcus neoformans, and basic peptides in body fluids have been shown to inhibit Candida albicans.
Results of clinical and experimental studies indicate that C. albicans, C. neoformans, Aspergillus fumigatus, and C. immitis activate the alternative pathway of the complement cascade. Because of the polysaccharide nature of fungal cell walls, it is expected that all medically important fungi activate complement. Such activation may be important in defense against some mycoses; a positive correlation has been demonstrated between animals deficient in late-acting complement components (C3-C9) and increased susceptibility to fungi such as C. neoformans and C. albicans. Assuming that phagocytic cells are important in resistance to fungi, complement activation may play a role by provoking an acute inflammatory response on generation of complement fragments C3a and C5a, and by coating the fungal elements with opsonic fragments C3b and C3d for ingestion by phagocytic cells.
The systemic mycoses of humans and other animals are caused by some fungi that are pathogenic and cause disease in the healthy host, and by other fungi (opportunistic pathogens) that are usually innocuous but cause disease in patients whose immune defenses are impaired. Some of these fungi may be saprophytes in nature (soil, bird droppings), whereas others are a part of the normal human flora (commensals). In no case are humans the solitary or necessary host.
An example of a soil saprophyte is Histoplasma capsulatum, which commonly causes infection in endemic areas; 80%-90% of adults react positively to histoplasmin in delayed cutaneous hypersensitivity tests. An example of an opportunistic pathogen is Candida albicans, normally present in the oral cavity, gastrointestinal tract, and probably the skin. In the patient with acute leukemia, however, C. albicans is commonly present in blood, causing a fulminant, usually fatal, septicemia. Other opportunistic infections are seen in patients with diabetic acidosis (mucormycosis) and Hodgkin's disease (for example, cryptococcosis and histoplasmosis). The pathogenesis of these mechanisms is obscure, but cell-mediated immunity seems to be essential for a good prognosis.
Neither active vaccines nor passive immune serum immunization has been sufficiently successful to result in commercially available preparations.
Treatment of active disease may be symptomatic (for example, pain relief), sometimes surgical (resection of irremedially damaged tissue and correction of hydrocephalus), and, most successfully, chemotherapeutic (Table 1). Among the chemotherapeutic agents commonly used are hydroxystilbamidine isethionate, amphotericin B, 5-fluorocytosine (Flucytosine), miconazole, and ketoconazole. Response to these drugs varies according to the fungus, type of disease, and course of illness. For example, response is good in most B. dermatitidis infections, but is poor in most diseases caused by A. fumigatus. Response is better for skin lesions caused by B. dermatitidis than for meningitis due to C. immitis; response is better in chronic cryptococcosis than in fulminant candidiasis. Table 1 shows a listing of some systemic mycoses and generally accepted chemotherapeutic agents.
TABLE 1 ______________________________________ CHEMOTHERAPEUTIC AGENTS FOR SYSTEMIC MYCOSES Disease First Choice Second Choice ______________________________________ Aspergillosis Amphotericin B Ketoconazole Blastomycosis Amphotericin B Hydroxystilbamidine isethionate Candidiasis Amphotericin B Flucytosine or ketoconazole Coccidioidomycosis Amphotericin B Ketoconazole Cryptococcosis Amphotericin B Either drug alone* Flucytosine Histoplasmosis Amphotericin B Ketoconazole* Mucormycosis Amphotericin B Miconazole* Paracoccidioidomycosis Amphotericin B Sulfonamides, Ketoconazole* ______________________________________ *Depending on minimal inhibitory concentration necessary for the fungus.
Infection is the cause of death in 51% of patients with lymphoma and 75% of patients with leukemia. Although bacteria are the causative organisms of many such infections, fungi account for 13% of the fatal infections in patients with lymphoma and for more than 20% of patients with leukemia. The fungus Candida albicans causes more than 80% of these infections, and Aspergillus spp. is also a frequent cause of such infections. In addition, fungal infection is a major cause of morbidity and mortality in patients with congenital and acquired deficiencies of the immune system. Much concerted effort has been expended in search of agents useful in treating fungal infections of humans. As a result, many compounds have been isolated and shown to have antifungal activity, but problems associated with solubility, stability, absorption, and toxicity have limited the therapeutic value of most of them in human infections. The most useful antifungal antibiotics fall into one of two categories: those that affect fungal cell membranes and those that are taken up by the cell and interrupt vital cellular processes such as RNA, DNA, or protein synthesis. Table 2 lists some useful antifungal agents and their mechanisms of action.
TABLE 2 ______________________________________ SOME USEFUL ANTIFUNGAL AGENTS, THEIR CHEMICAL CLASSIFICATION, AND THEIR MECHANISMS OF ACTION Class Compounds Mechanism ______________________________________ Polyene Amphotericin B Interacts with sterols Nystatin (ergosterol) in fungal cell membrane, render ing cells selectively permeable to the outflow of vital constituents, e.g. potassium Imidazole Miconazole Inhibits demethylation of Clotrimazole lanosterol thus Ketoconazole preventing formation of ergosterol, a vital component of fungal cell membrane; also has a direct cidal effect on fungal cells Pyrimidine 5-Fluorocytosine Is taken up and deaminated by susceptible cell to form 5-fluorouracil, which in turn inhibits RNA synthesis; also thought to inhibit thymidylate synthetase and DNA synthesis Grisan Griseofulvin Binds to tubulin and inhibits microtubule assembly 3-Arylpyrrole Pyrrolnitrin Appears to inhibit terminal electron transport between succinate or NADH and coenzyme Q Glutaramide Cycloheximide Inhibits protein synthesis at 80S ribosomal level preventing transfer of aminoacyl tRNA to the ribosome ______________________________________
The polyene macrolide antibiotics are secondary metabolites produced by various species of Streptomyces. Several common features of these compounds are useful in classifying the more than 80 different polyenes that have been isolated. All are characterized by a macrolide ring, composed of 26-38 carbon atoms and containing a series of unsaturated carbon atoms and hydroxyl groups. These features of the molecule contribute to the polyenes' amphipathic properties (those relating to molecules containing groups with different properties, for example, hydrophilic and hydrophobic). The ring structure is closed by the formation of an internal ester or lactone bond (FIG. 1). The number of conjugated double bonds vary with each polyene, and the compounds are generally classified according to the degree of unsaturation.
Toxic effects of polyene macrolides appear to be dependent on binding to cell membrane sterols. Thus, they bind to membranes of fungus cells as well as to those of other eukaryotic cells (human, plant, and protozoa), but not to bacterial cell membranes, which do not contain membrane sterols. The interaction of polyene macrolides with mammalian and fungal membrane sterols results in transmembrane channels that allow the leakage of intracellular components leading to cell deaths.
The usefulness of an antibiotic is usually measured by the differential sensitivity of the pathogen and host. Two polyene macrolides agents, nystatin and amphotericin B, are relatively specific for fungi and have thusfar proven to have therapeutic usefulness in humans. The relative specificity of these two polyene macrolides may be based on their greater avidity for ergosterol, the principal sterol of fungal membranes, compared to cholesterol, the principal sterol of human cell membranes.
Amphotericin B is a heptaene macrolide with seven resonating carbon bonds. The compound was first isolated from broth filtrates of S. nodosum in 1956. Like other polyene macrolide antibiotics, amphotericin B is insoluble in water. The problem of its solubility has been circumvented by combining the antibiotic with sodium deoxycholate and sodium phosphate and hydrating the mixture with sterile water or saline. Amphotericin B is the polyene antibiotic thusfar most sufficiently nontoxic to humans that it has been used parenterally at effective doses against various fungi.
Nystatin, first isolated from S. noursei, is structurally related to amphotericin B, but is not classified as a heptaene because the conjugated portion of the ring is interrupted and thus forms a tetraene and a diene. Tolerated well both orally and topically, the drug is not available for intravenous use because of its presumed high toxicity and aqueous insolubility. Nystatin is available as oral tablets (500,000 units) or as an ointment for topical use (100,000 units/g). It is used in the management of cutaneous and mucocutaneous candidiasis.
It has recently been shown that the encapsulation of certain drugs in liposomes before administration to the patient can markedly alter the pharmacokinetics, tissue distribution, metabolism and therapeutic efficacy of these compounds. Liposomes may be defined as lipid vesicles which are formed spontaneously on addition of an aqueous solution to a dry lipid film. Further, the distribution and pharmacokinetics of these drugs can be modified by altering the lipid composition, size, charge and membrane fluidity of the liposome in which they are encapsulated.
Recently, liposomes have been used as carriers of amphotericin B for treatment of murine leishmaniasis (New, R.R.C., et al., "Antileishmanial Activity of Amphotericin and Other Antifungal Agents Entrapped in Liposomes." J. Antimicrob. Chemother., Vol. 8 (1981), pp. 371-381), histoplasmosis (Taylor, R.L., et al., "Amphotericin B in Liposomes: A Novel Therapy for histoplasmosis." Am. Rev. Respir. Dis., Vol. 125 (1982), pp. 610-611), cryptococosis (Graybill, J.R., et al., "Treatment of Murine Cryptococosis with Liposome-Associated Amphotericin B." J. Infect. Dis., Vol. 145 (1982), pp. 748-752). and candidiasis (Tremblay, C., et al., "Comparative Efficacy of Amphotericin B (AMB) and Liposomal AMB (lip-AMB) in Systemic Candidiasis in Mice." Abstr. 1983 ICAAC, No. 755 (1983), p. 222). Liposome-encapsulated Amphotericin B has also been used for treatment of coccidioidomycosis in the Japanese macaque (Graybill, J.R., et al., "Treatment of Coccidioidomydosis (cocci) in Primates Using Liposome Associated Amphotericin B (Lipo-AMB)." Abstr. 1982 ICCAC, No. 492 (1982), p. 152).
The treatment of fungal infections remains a major problem in spite of the availability of effective antifungal drugs such as the polyenes. Most of the available polyene antibiotics have toxic side effects that limit their clinical application. Nystatin, a tetraene-diene polyene macrolide antibiotic, has high hydrophobicity, which has precluded its effective systemic administration. It has been used as suspensions prepared in various ways and administered to the patients orally. However, these studies have generally failed to document a beneficial effect of nystatin administration against systemic fungal infections.
The present inventors have recently demonstrated that liposome-encapsulated amphotericin B may be used to treat experimental murine candidiasis (Lopez-Berestein et al., J. Infect. Dis., Vol. 150, pp 278-283 (1984) and in the treatment of fungal infections in patients with leukemia and lymphoma (Lopez-Berestein et al., J. Infect. Dis., Vol. 151, pp 704-71 (1985).